A gear made of a resin is conventionally used in a power-transmitting mechanism such as a duplicator, an automobile part and the like for the purpose of reducing the cost and the weight of the part and reducing an operational sound. Such a gear made of a resin is formed into a predetermined shape by an injection molding, and the shape is contrived to provide a tooth-profile accuracy and a strength suited for the purpose of use.
For example, in an image-forming device such as a color duplicator, a gear made of a resin adapted to be meshed at a high contact rate with a gear made of a resin for driving a photoconductor is used and the shape of this gear is contrived for forming a distinct color image having a high quality. The gear made of the resin is connected to a photoconductor-driving motor (a drive means), so that the rotational force of the photoconductor-driving motor is transmitted smoothly through the gear made of the resin to a photoconductor to turn the photoconductor smoothly and with a high accuracy, thereby effectively preventing a printing failure such as a color shift.
(First Prior Art)
For example, a gear 150 made of a resin shown in FIGS. 1 and 2 is formed into such a shape that an annular toothed portion 152 having teeth 151 formed thereon and a boss (shaft-supporting section) 154 fitted over a shaft 153 are connected to each other by a thin plate-shaped web 155. The web 155 connected to the toothed portion 152 is thinner and hence, the amount of radially inward contractive deformation (shrinkage transformation) of a connection between the web 155 and the toothed portion 152 after the injection molding and a portion in the vicinity of the connection is smaller, whereby a tooth profile can be formed with a higher accuracy. When the thickness of the web 155 connected to the toothed portion 152 is larger, the volume of the web 155 is larger, and the amount of radially inward contractive deformation of the web 155 after the injection molding is larger. For this reason, the connection between the web 155 and the toothed portion and the portion in the vicinity of the connection are deformed in such a manner that they are pulled in a direction of contraction of the web 155 and hence, the amount of toothed portion 152 deformed is larger, resulting in an increased reduction in accuracy of formation of the teeth 151.
On the other hand, when the gear 150 made of the resin as shown in FIGS. 1 and 1 transmits a power in a state in which it has received a radial load WR as shown in FIG. 3, the web 155 is deformed elliptically in an increased amount, as shown by a dotted line in FIG. 3, resulting in a decreased accuracy of transmission of the rotation, because the web 155 receiving the radial load WR has a smaller thickness and a lower strength. When the gear 150 having a sectional shape as shown in FIG. 2 is a helical gear, a thrust load WS is applied to the toothed portion 152 and the web 155 and hence, the web 152 is deformed in a flexed manner in an increased amount in a direction of application of the thrust load WS, as shown by a dotted line in FIG. 4, resulting in a reduction accuracy of transmission of the rotation, because the thickness of the web 155 is smaller.
(Second Prior Art)
Therefore, to eliminate such a disadvantage, a gear 157 made of a resin has been considered which includes radial ribs 156 formed on a side of a web 155 to connect a boss 154 and a toothed portion 152 to each other, as shown in FIGS. 5 to 7, so that the rigidity of the web 155 is increased, and the accuracy of transmission of the rotation is enhanced. In the gear 157 in which the boss 154 and the toothed portion 152 are connected directly to each other by the radial ribs 156, however, the following new disadvantage is brought about: A connection between the radial ribs 156 and the toothed portion 152 and a portion in the vicinity of the connection are contractively deformed radially inwards in an increased amount (shown by a dotted line in each of FIGS. 5 and 6), thereby generating a phenomenon of the falling of the teeth 151 as shown in FIG. 8, resulting in a reduced accuracy of a tooth profile.
(Third Prior Art)
A gear 200 made of a resin as shown in FIGS. 9 and 10 has been developed by the present applicant, wherein such disadvantages associated with the prior arts can be overcome.
The gear 200 made of the resin shown in FIGS. 9 and 10 includes a shaft-supporting section 203 having a shaft hole 202 fitted over a photoconductor driving shaft 201 for rotation in unison with each other, a rim 205 having teeth 204 and located radially outside the shaft-supporting section 203, and a thinner web 206 which connects the shaft-supporting section 203 and the rim 2O5. In order to prevent the deformation of the web 206 due to a thrust load, this gear 200 made of the resin has first and second annular circumferential ribs 207 and 208 formed inside the rim 205 and on opposite sides of the web 206, diametrical ribs 210 formed radiately on opposite sides of the web 206 between the shaft-supporting section 203 and the second circumferential rib 208 to connect the shaft-supporting section 203 and the second circumferential rib 208 to each other, and diametrical ribs 211 formed radiately on the opposite sides of the web 206 between the first and second circumferential rib 207 and 208 to connect the first and second circumferential rib 207 and 208 to each other.
In the gear 200 made of the resin and having such configuration, the diametrical ribs 210 and 211 are not connected to the rim 205 and hence, the rigidity of the web 206 can be increased without detraction of the roundness of the rim 205 having the teeth 204. The gear 200 made of the resin is formed into a shape shown in FIGS. 9 and 10 by an injection molding and hence, as the thickness of the resin is larger, a longer time is taken to cool the resin within a cavity, and the amount of resin contractively deformed is increased more. Therefore, in the gear 200 made of the resin having such shape, if the thickness of the web 206 is larger, the thickness of the connection between the web 206 and the rim 205 is larger, and the amount of radially inward contractive deformation of the connection between the web 206 and the rim 205 is larger than those of other portions, resulting in a degraded tooth profile accuracy. If the diametrical ribs 212 are connected to the rim 213, as shown in FIG. 11, the thickness of the connection between the diametrical ribs 212 and the rim 213 is larger, and the amount of radially inward contractive deformation of the connection between the diametrical ribs 212 and the rim 213 is larger than those of other portions (see a portion shown by a dotted line in FIG. 11), resulting in a reduced roundness. Therefore, the conventional gear 200 made of the resin shown in FIGS. 9 and 10 is formed so that the thickness of the web 206 is decreased as much as possible to provide a desired accuracy of the teeth 204, and the lack of the rigidity of the web 206 can be compensated for by the first and second circumferential ribs 207 and 208 and the diametrical ribs 210 and 211.
In recent years, in an image-forming device such as a color printer, a color duplicator and the like, a small error of the rotation-transmitting accuracy of each of the gears 157 and 200 made of the resin appears as a color shift. Therefore, to prevent the color shift to enable a further clear or distinct printing, it is necessary to rotate the photoconductor more smoothly and with a higher accuracy than those in the prior art to enhance the accuracy of formation of an image on the photoconductor. Here, what exerts a large influence to the accuracy of rotation of the photoconductor is the accuracy of the gear made of the resin, as described above.
In the conventional gear 200 made of the resin, however, the following has been made clear: The rim 205 having the teeth 204 is deformed due to a torque applied during transmission of a power, so that it is displaced in a rotational direction with respect to the shaft-supporting section 203. Particularly, the deformation of a portion between the shaft-supporting section 203 and the second circumferential rib 208 is larger than those of other portions, and the diametrical ribs 210 formed around the outer periphery of the shaft-supporting section 203 are deformed as shown by the dotted line. For this reason, a lag is produced between the rotation transmitted from the photoconductor-driving motor (not shown) to the helical gear 200 made of the resin and the rotation transmitted from the gear 200 made of the resin to the photoconductor-driving shaft 201, and a reduction in an image quality such a color shift and the like is brought about due to the lag between the rotations.
The recent image-forming device such as a color printer, a color duplicator and the like is disposed and used beside a desk in many cases and for this reason, it is required that the image-forming device is designed so that a vibration during operation and a noise due to the vibration can be reduced to maintain a quiet environment in a working place.